The present invention relates to mass spectrometers.
Time of flight mass analysers are discontinuous devices in that they receive a packet of ions which is then injected into the drift region of the time of flight mass analyser by energising a pusher/puller electrode. Once injected into the drift regions, the ions become temporally separated according to their mass to charge ratio and the time taken for an ion to reach a detector can be used to give an accurate determination of the mass to charge ratio of the ion in question.
Many commonly used ion sources are continuous ion sources such as Electrospray or Atmospheric Pressure Chemical Ionisation (“APCI”). In order to couple a continuous ion source to a discontinuous time of flight mass analyser an ion trap may be used. The ion trap may continuously accumulate ions from the ion source and periodically release ions in a pulsed manner so as to ensure a high duty cycle when coupled to a time of flight mass analyser.
A commonly used ion trap is a 3D quadrupole ion trap. 3D quadrupole ion traps comprise a central doughnut shaped electrode together with two generally concave endcap electrodes with hyperbolic surfaces. 3D quadrupole ion traps are relatively small devices and the internal diameter of the central doughnut shaped electrode may be less than 1 cm with the two generally concave endcap electrodes being spaced by a similar amount. Once appropriate confining electric fields have been applied to the ion trap, then the ion containment volume (and hence the number of ions which may be trapped) is relatively small. The maximum density of ions which can be confined in a particular volume is limited by space charge effects since at high densities ions begin to electrostatically repel one another.
It is desired to provide an improved ion trap, particularly one which is suitable for use with a time of flight mass analyser.